Electronic fuel control systems are increasingly being used in internal combustion engines to precisely meter the amount of fuel required for varying engine requirements. Such systems vary the amount of fuel delivered for combustion in response to multiple system inputs including throttle angle and the concentration of oxygen in the exhaust gas produced by combustion of air and fuel.
Electronic fuel control systems operate primarily to maintain the ratio of air and fuel at or near stoichiometry. Electronic fuel control systems operate in a variety of modes depending on engine conditions, such as starting, rapid acceleration, sudden deceleration, and idle. One mode of operation is known as closed-loop control. Under closed-loop control, the amount of fuel delivered is determined primarily by the concentration of oxygen in the exhaust gas, the oxygen concentration being indicative of the ratio of air and fuel that has been ignited.
The oxygen in the exhaust gas is sensed by a Heated Exhaust Gas Oxygen (HEGO) sensor. The electronic fuel control system adjusts the amount of fuel being delivered in response to the output of the HEGO sensor. A sensor output indicating a rich air/fuel mixture (an air/fuel ratio below stoichiometry) will result in a decrease in the amount of fuel being delivered. A sensor output indicating a lean air/fuel mixture (an air/fuel ratio above stoichiometry) will result in an increase in the amount of fuel being delivered.
Modern automotive engines utilize a three-way catalytic converter to reduce the unwanted by-products of combustion. The catalytic converter has a finite number of active sites where the electronic forces are optimum for an electrochemical reaction to take place. The number of active sites limits the mass quantity of reactants that the converter is able to process at any given time.
Maintenance of the ratio of air and fuel at or near stoichiometry is critical to efficient operation of the catalytic converter. In order to affect a maximum conversion efficiency from a three-way catalyst, discrete cyclical quantities of rich and lean exhaust gases must be delivered to the catalyst. Balancing the excursions between rich and lean exhaust gases is important in ensuring that an adequate number of active sites in the converter are available for conversion to take place. A lean air/fuel excursions will oxidize the active sites leaving the ensuing rich excursions to reduce the active sites. In this manner, by alternately processing rich and lean mixtures, the catalytic converter will attain maximum conversion efficiencies. The magnitude and frequency of the rich/lean excursions, however, should never be large enough to saturate the catalyst. A calibration that is either too rich or too lean will cause saturation of the catalyst. The frequency of these excursions will vary with engine operating speed and/or load conditions. Proper control of these necessary excursions increases the efficiency of the converter, thus leading to lower tailpipe emissions.
When altering the air/fuel ratio in response to the detected exhaust gas oxygen content, electronic fuel control systems known in the art respond in a predetermined way to a detected fuel ratio. Consequently, factors such as imprecision in the predetermined response, variation from engine to engine, aging of parts and changes in operating conditions will be unaccounted for, and the performance and efficiency of the engine will suffer accordingly.